1. Field of the Invention
The present invention relates to a device for detecting a fine displacement of a target by utilizing the reflecting rays and more specifically to an improvement of the detector which, by using optical fibers, detects the fine displacement of the target from the intensity of reflected light.
2. Description of Prior Art
An instrument is known which uses optical fibers, allowing a fine displacement of a few of microns to be detected in a noncontact manner. (See R. O. Cook and C. W. Hamm "Fiber Optic Lever Displacement Transducer") The measuring principle is hereinafter described with reference to FIG. 10. In FIG. 10, two optical fibers are provided in such a way that fibers are adjacent end- to end each other. Of two optical fibers, the light from the light source 2 is made incident to one end of an illuminating fiber 1, while a cone like beam of illuminating light 3 extending from the other end is made to irradiate. When a reflection surface 5 is placed by a distance spaced apart from an irradiance plane 4 of fiber 1, the illuminating light beam 3 is reflected and a part of the light is made incident to the end surface of the light receiving optical fiber 6. Hereby a zone 7 which is illuminated on the reflection surface 5, as shown in FIG. 10, takes a rectangular form, while a zone 8 where the light receiving fiber 6 can receive the light also takes the same rectangular form. Consequently, of the zone 7 occupied by the illuminating light, the zone where rays are reflected and come into the light receiving optical fiber 6 occupies a hatched part 9 only. Since the illuminating intensity and the size of area of this hatched part 9 is varied in correspondence to the distance d, the value of distance can be known by detecting the amount of rays coming into the light receiving optical fiber 6 by means of a photodetector 10 provided at the other end of the optical fiber. Example of relationship between this distance d and the intensity Ip of the light receiving signal is shown in FIG. 11. In FIG. 11, the intensity of light Ip is increased substantially linearly with the increase of distance d, saturated at a certain point and is thereafter inversely decreased. The distance dm where the intensity of light Ip becomes the greatest depends on the radius and the numerical apertures of the optical fiber, amounting approximately to a value 1.5 times as great as the radius of the optical fiber. When the optical fiber having a radius of about 50 .mu.m is used, a fine displacement of about 0 to 30 .mu.m can be detected substantially linearly with the resolution above 0.1 .mu.m.
However, according to the above-mentioned method, as seen from the detecting principle, when the intensity of source of light 2 is fluctuated or the reflection coefficient of reflection surface 5 is varied, since the intensity of light Ip is varied as shown in FIG. 11 by the broken line with respect to the displacement (distance d), the detecting sensitivity of displacement must be corrected each time the ditection is made. Namely, in prior devices, it has been indispensable to correct the intensity of light source 2 and the reflection coefficient of the reflection surface 5 just prior to their use, which has made the operation extremely complicated.
Besides, in prior devices, in case the reflection coefficient of reflection surface is unknown, it has been necessary to previously obtain the relationship between the displacement and the light intensity Ip to obtain knowledge of the reflection coefficient in advance and to proceed to the above-mentioned manipulation.
Further, in prior devices, in case the reflective target plane is moved in the direction other than the measuring direction and the reflection coefficient of the reflective target plane to be detected is not uniform but is varied little by little, the detected result can include two adverse factors, the varied displacement and the varied reflection coefficient, as the result of which, if the displacement is sought directly from the detected result, measuring errors could arise.
An object of the present invention is therefore to eliminate disadvantages caused in the above prior art and to provide a fine displacement transducer which allows the displacement to be detected without undergoing the adverse effect caused by the varied reflection coefficient of reflective surface or the fluctuation of intensity of light source.